import Data.List ((!!))
import Control.Monad.Trans.Except
module SafeList where

data ListIndexError = ErrIndexTooLarge Int | ErrNegativeIndex
  deriving (Eq, Show)

infixl 9 !!!

isNonEmpty [] = False
isNonEmpty (_:_) = True

longerThan :: Int -> [a] -> Bool
longerThan n xs = isNonEmpty $ drop n xs

(!!!) :: [a] -> Int -> Except ListIndexError a
(!!!) lst ind | ind < 0           = throwE $ ErrNegativeIndex
              | longerThan (ind + 1) lst = return $ lst !! ind
              | otherwise         = throwE $ ErrIndexTooLarge ind

(!!!!) xs n = runExcept $ xs !!! n

{-
GHCi> runExcept $ [1..100] !!! 5
Right 6
GHCi> [1,2,3] !!!! 0
Right 1
GHCi> [1,2,3] !!!! 42
Left (ErrIndexTooLarge 42)
GHCi> [1,2,3] !!!! (-3)
Left ErrNegativeIndex
GHCi> [1..] !!!! 0
Right 1
-}